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Lecture 2

Physiology 3120 Lecture Notes - Lecture 2: Hydrophile, Intrapleural Pressure, Pneumothorax

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Tom Stavraky

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Physiology 3120
Dr. Veldhuizen
Ventilation and Lung Compliance
- Process by which air moves in and out the lung
- You expand your ribcage by contracting your diaphragm and some of the muscles
- the process of ventilation is very similar to this example:
o bottle that is airtight with a balloon that is connected to the air with a membrane at the
o if you pull the membrane, you tank the pressure in the glass bottle (make it more ive)
o to compensate for that, air will flow into the balloon and it will expand
- Active process (contraction of the inspiratory muscles, which are…
o The diaphragm
o The external intercostals
- Increase pressure gradient (making it more ive) between lung and
intrapleural space so that air can flow into the lunginflation
- Normal a passive process
o Lung and chest wall return to their equilibrium position
- Active process during exercise or spontaneous hyperventilation
o Muscles of the abdominal wall
o The interal intercostals contract to bring the ribcage in
Lung Compliance
- Doesn’t take much effort to inhale air
o We create a relatively small change in pressure by opening up our thoracic cavity to
- Despite the fact that we have a very large SA in the lung
o During inhalation, we try to expand the SA and open it up for the blood-gas barrier to
have gas exchange
o So it’s a large area but it doesn’t take much pressure to do this
- This is due to lung compliance
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- Lung compliance = volume change per unit pressure change
o due to the distensibility of the lung
- Termed for this property lung compliance (or pulmonary compliance)
Question: At a lung physiology conference, two scientists get into a bit of an argument. After a while
things get out of hand and scientist 1 yells at scientist 2 One more word out of you and I’ll poke a hole in
your diaphragm. Oh yeah says Scientist 2 if you don’t stop I am gonna cut all your internal
If these scientists do to each other what they are threatening to do: What will happen to them, whose
lung function will be more affected and why...
o If you cut the diaphragm, you create pneumothorax (cant inflate your lungs)
o If you cut the internal intercostals, you only affect active exhalation you will still be able to
passively exhale
o Scientist 1One more word out of you and I’ll poke a hole in your diaphram.
o Result: Intrapleural pressure will be atmospheric... no inflation
o Scientist 2 If you don’t stop I am gonna cut all your internal intercostals
o Result: Internal intercostal are only need for active expiration,
o normal expiration (and inspiration) will still occur
Lung Compliance
- lung compliance can be determined by creating a pressure-volume curve
Pressure Volume Curve
- to create a PV curve, you want to change to pressure and measure
the volume or vice versa
- you take out the lung of an animal
- you attach a syringe where you can inflate and deflate that lung
- at the same time, you can measure the pressure in the system
- often in PV curve, we measure pressure in cm H2O but in other
pressure curves we measure pressure in mmHg its just a
practical thing
Start of Inflation
- when you start injecting a little bit of volume, your pressure jumps up by about 2 cm H2O
- so you don’t actually get that much volume in the lung at the initial stage
- but even with a small amount of volume, you rapidly increase the pressure
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Continuation of Inflation
- when you continue the process, bw 8-10 cmH2O, the lungs all of the sudden open up and take a
lot more volume for a given pressure interval
- if you go 10-12 cm H2O of pressure, it takes a lot of volume to inflate that lung
- so in this region, the lung becomes more compliant
End of Inflation
- you continue this process, and at some point, the lungs are completely filled and even if you
tried to put more air into it, you only increase the pressure
- so at the end of the PV curve, the curve kind of flattens out (at around 20-25 cm H2O)
- where the curve flattens out varies bw species doesn’t vary by that much though but the shape
of the curve is still the same
o ex: in a rat lung, your volumes will be a lot lower, bc its smaller but the pressures will be
about the same ranges
- when you deflate the lung, the curve looks different
- initially, when you start to deflate the lung, it maintains a lot of volume, even for small changes
in pressure
o at the upper part the lung is not that compliant for changes in pressure, there is not
that much change in volume
- then it rapidly increases, and you get back at 0 pressure
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